All areas of geologic study are subdisciplines of either historical geology, which focuses on the chemical, physical, and biological history of Earth, or physical geology, which is the study of Earth materials and processes. Historical geology uses theory, observation, and facts derived from studying rocks and fossils to learn about the evolution of Earth and its inhabitants.

According to the principle of uniformitarianism, most physical and chemical processes occurring today are very similar to those that operated in the geologic past, although their rates may be different. Therefore, by studying modern geologic activities and their products, geologists can understand how these activities produced the ancient rock record. In other words, the present is the key to the past. The principle of uniformitarianism has been very useful in deciphering much of the rock record.

Studies in historical geology rely on the rock record for factual information about...